Chemical vapour deposition (CVD) processes for synthesis of diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R. S Balmer et al. gives a comprehensive overview of CVD diamond materials, technology and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).
Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, synthetic diamond material can be deposited.
Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma.
Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas stream and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantage of being restricted to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface.
In light of the above, it has been found that microwave plasma is the most effective method for driving CVD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable.
A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur.
A range of possible microwave plasma reactors for synthetic diamond film growth using a CVD process are known in the art. Such reactors have a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder.
A useful overview article by Silva et al. summarizing various possible reactor designs is given in the previous mentioned Journal of Physics (see “Microwave engineering of plasma-assisted CVD reactors for diamond deposition” J. Phys.: Condens. Matter, Vol. 21, No. 36 (2009) 364202). Having regard to the patent literature, U.S. Pat. No. 6,645,343 (Fraunhofer) discloses an example of a microwave plasma reactor configured for diamond film growth via a chemical vapour deposition process. The reactor described therein comprises a cylindrical plasma chamber with a substrate holder mounted on a base thereof. A cooling device is provided below the substrate holder for controlling the temperature of a substrate on the substrate holder. Furthermore, a gas inlet and a gas outlet are provided in the base of the plasma chamber for supplying and removing process gases. A microwave generator is coupled to the plasma chamber via a high-frequency coaxial line which is subdivided at its delivery end above the plasma chamber and directed at the periphery of the plasma chamber to an essentially ring-shaped microwave window in the form of a quartz ring mounted in a side wall of the plasma chamber.
Using microwave plasma reactors such as those disclosed in the prior art it is possible to grow polycrystalline diamond wafers by chemical vapour deposition on a suitable substrate such as a silicon wafer or a carbide forming refractory metal disk. Such polycrystalline CVD diamond wafers are generally opaque in their as-grown form but can be made transparent by polishing opposing faces of the wafers to produce transparent polycrystalline diamond windows for optical applications.
Diamond material is useful as an optical component as it has a broad optical transparency from ultraviolet through to infrared. Diamond material has the additional advantage over other possible window materials in that it is mechanically strong, inert, and biocompatible. For example, the inertness of diamond material makes it an excellent choice for use in reactive chemical environments where other optical window materials would not be suitable. Further still, diamond material has very high thermal conductivity and a low thermal expansion coefficient. As such, diamond material is useful as an optical component in high energy beam applications where the component will tend to be heated. The diamond material will rapidly conduct away heat to cool areas where heating occurs so as to prevent heat build-up at a particular point, e.g. where a high energy beam passes through the material. To the extent that the material is heated, the low thermal expansion coefficient of diamond material ensures that the component does not unduly deform which may cause optical and/or mechanical problems in use.
One problem with fabricating polycrystalline CVD diamond optical components is that during the CVD growth process impurities such as nitrogen, silicon and non-diamond carbon are incorporated into the diamond material as discussed below.
Atmospheric nitrogen is generally present as an impurity within source process gases and may also be present as a residual impurity within CVD reactor components due, for example, to non-perfect vacuum seals and/or residual impurities adsorbed onto interior surfaces of the CVD reactor which may desorb during use. Furthermore, nitrogen gas is often intentionally introduced into the CVD synthesis atmosphere during a synthetic diamond growth process as it is known that nitrogen increases the growth rate of synthetic diamond material. While nitrogen is advantageous for achieving commercially useful growth rates, incorporation of nitrogen into the synthetic diamond material can detrimentally affect the optical and thermal performance characteristics of the material. Accordingly, a balance may be struck between providing sufficient nitrogen within the CVD synthesis atmosphere to achieve acceptable growth rates while limiting the quantity of nitrogen which is incorporated into the solid CVD diamond material being grown. Apparatus and process conditions can affect the rate at which nitrogen within the CVD synthesis atmosphere is incorporated into the solid CVD diamond material being grown.
Silicon impurities may come from silicon based components within the CVD reactor. For example, quartz windows or bell jars are often used to couple microwaves into the plasma chamber and/or constrain plasma and process gases near a substrate growth surface to achieve CVD diamond growth. Such silicon containing quartz components are exposed to extreme temperatures from the plasma in use and this can result in silicon from these components being incorporated into the synthetic diamond material. Apparatus and process conditions can affect the rate at which silicon is incorporated into the solid CVD diamond material being grown.
Non-diamond carbon (e.g. sp2 hybridized graphitic carbon) is inevitably deposited on the growth surface of the substrate during CVD diamond growth processes. As previously described, atomic hydrogen is essential to a CVD diamond growth process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. However, this selective etching process does not usually remove all the deposited non-diamond carbon and such material therefore becomes incorporated into the CVD diamond material. Apparatus and process conditions can affect the rate at which non-diamond carbon is incorporated into the solid CVD diamond material being grown.
In light of the above, it is evident that the apparatus configuration and process conditions must be carefully selected and controlled in order to ensure that the level of impurities incorporated into the synthetic diamond material during CVD growth are extremely small for high performance optical components.
In addition to control of absolute impurity levels, it is also critical to ensure that the uniformity of impurity uptake is controlled so as to achieve a product which has consistent performance characteristics. Uniformity is an issue in terms of spatial variations in the rate of impurity uptake across a growth surface and temporal variations in the rate of impurity uptake over a growth run. For example, a non-uniform distribution of physical and/or chemical process parameters over the growth surface can lead to spatial variations in the rate of impurity uptake across a synthetic polycrystalline diamond wafer. Furthermore, as a synthetic polycrystalline diamond wafer grows, grain boundaries within the synthetic polycrystalline diamond wafer increase in size. An increase in the size of grain boundaries as the synthetic polycrystalline diamond wafer grows thicker leads to an increase in the rate of impurity uptake within the enlarged grain boundaries which can result in an increasing concentration of impurities through the thickness of a synthetic polycrystalline diamond wafer.
In addition to the above described problems, variations in growth rate across a synthetic polycrystalline diamond wafer can lead to variations in impurity uptake. For example, as the growth rate increases the time available to etch non-diamond carbon from the growth surface before it is encapsulated within the synthetic polycrystalline diamond wafer decreases. Furthermore, variations in growth rate also cause variations in thickness which can lead to strain and cracking of synthetic polycrystalline diamond wafer on cooling after completion of the CVD growth process. Variations in growth rate can be caused by non-uniformities in the plasma across the growth surface and non-uniformities in the temperature of the substrate on which the synthetic polycrystalline diamond wafer is grown.
Despite the above problems, to date it has been possible to fabricate high optical quality polycrystalline diamond wafer up to approximately 100 mm in diameter. However, the production of larger polycrystalline diamond wafers of high optical quality has proved problematic. While it has been possible to fabricate larger polycrystalline diamond wafers, these have been of lower optical quality, particularly towards the periphery of the wafers. Such wafers do not meet the requirements for certain commercial applications which require relatively thick, relatively large diameter synthetic polycrystalline diamond windows of extremely high optical quality. For example, certain very high powered laser beam applications require >120 mm diameter clear aperture, optical grade, polycrystalline diamond laser windows capable of handling the extreme power densities involved. Polycrystalline diamond laser windows with the relevant optical properties are available in smaller sizes. However these sizes are not large enough for certain applications. Such polycrystalline diamond windows are also required for use as radiation resistant windows.
It is an aim of certain embodiments of the present invention to provide a suitable microwave plasma reactor configuration and suitable CVD process conditions in order to fabricate large (e.g. at least 120 mm diameter) synthetic polycrystalline diamond windows having extremely high optical quality across substantially all (e.g. at least 80%) of the window area.